This invention relates to a device for measuring temperature in molten metals. The device contains a ceramic tube that is closed on one side, a thermocouple arranged in the tube, and an external protective body that surrounds the tube, forming an annular space between the outer surface of the tube and the inner surface of the protective body.
A device of this type is known from GB-A-2193375. This document describes a thermocouple surrounded by a metal tube, in particular one made of molybdenum. In addition, the molybdenum tube is surrounded by an oxidic coating. Further devices of this type are known from EP 558808B1, which describes the advantage of filling the annular space between a protective body and a thermocouple tube.
Similar devices are known, for example, from German utility model GM 74 19 633. The arrangement described therein contains a tube that is closed on one side and surrounds a thermocouple inserted into a protective body made of heat-resistant metal oxide and graphite, whereby an annular gap is formed between the tube that is closed on one side and the protective body. For operation at the high temperatures that are common in molten metals, the thermocouple arranged in the tube is made of expensive materials, such as platinum. However, in continuous temperature measurements, which are required for continuous control of the molten metal, the thermocouple is insufficiently protected on the inside of this arrangement because reactive gases and/or corrosive gases, in particular, penetrate through the shells surrounding the thermocouple and thus can destroy the shells and the thermocouple. For this reason, it may be necessary to replace the thermocouple several times. Also, exposed to the high temperatures in the molten metal, silicon monoxide and carbon monoxide, for example, form inside the protective body of the device and penetrate through the aluminum oxide tube surrounding the thermocouple. The carbon monoxide increases the porosity of the tube, which slowly destroys the tube. Further, the silicon monoxide reacts with the thermocouple wire, effectively destroying the wire and rendering the temperature measurement incorrect and impossible to conduct.
Another device of the type described above is known from GB-B 2 193 375. The thermocouple in this device is surrounded by a ceramic-coated impermeable molybdenum tube that is closed on one side and is embedded in the protective body by means of a ceramic mass. This molybdenum tube is very expensive and laborious to produce. In order to prevent destruction due to thermomechanical tension, the molybdenum tube is surrounded by multiple ceramic layers having molybdenum contents that decrease from inside to outside. One of the purposes of these layers is to compensate for temperature gradients along the molybdenum tube. However, these ceramic layers and the molybdenum tube may also be destroyed by reactive gases, such as carbon monoxide or silicon monoxide.